1. Field of the Invention
The present invention relates to microlithographic fabrication, and more particularly to fabricating thick coplanar metallic microwave guides and electrodes.
2. Description of the Related Art
Thick (.gtoreq..apprxeq.5 .mu.m) metallic coplanar microwave electrodes and waveguides are desired for a number of applications, including resonant devices, nonresonant devices, and microwave/optical I/O modulators.
It is desired to make these structures arcuate or annular. It is also desired to make these structures with consistent sidewall angles.
In the case of microwave/optical I/O modulators, it is desired to provide good microwave-optical propagation velocity matching in the modulator. A microwave/optical I/O modulator is a device for converting a microwave signal into an optical signal. It is useful in microwave signal processing, since optical signals are generally simpler to work with than microwave signals. These modulators include a microwave guide and an optical waveguide, positioned for coupling between the two waveguides to provide signal modulation from one waveform to the other waveform. To achieve a high bandwidth for these modulators, it is necessary to achieve good matching of the propagation velocities for the microwave and optical waveforms in their respective waveguides.
The velocity of microwaves in a metallic coplanar guide is highly sensitive to the thickness and sidewall angle of the guide. Ideally, the sidewall angles of these electrodes are consistent, so that the propagation velocity will be governed only by the electrode thickness. However, making thick (.gtoreq..apprxeq.10 .mu.m) metallic structures with consistent sidewall verticality has proven to be difficult.
Problems with making these structures by photolithography include cracking in the thick photoresist layer, achieving full exposure throughout the photoresist layer, lift-off, and thermal flow degrading the photolithographic image.